Ferrites are ferrimagnetic iron oxides which exhibit spontaneous magnetization, however with magnetic response lower than ferromagnetic materials such as the transition metals. They are crystallographically shared in two main groups: cubic and hexagonal. In the group of cubic ferrites the general chemical formula is of the type: MO.Fe2O3. The group of hexagonal ferrites has the formula MOFe12O18 where M is a metal in a positive oxidation state, which can be Ba, Sr, Zn, Cu, Ni, Co, Fe, or Mn, or even a combination of those. The main part of the ferrofluids described in the literature are produced already in the liquid form, which hampers the control of the solids loading in the solution, obligates to transport all the liquid medium of the dispersion to the local of use and limits the changing of the solvents in the composition of the same. Furthermore, the concentrations of solids informed are low, in general, around 6% by volume.
Most magnetic particles that pass through the stage of formation of the powder depend on the exposure of the material to high temperatures, i.e. temperatures higher than 250° C. The processes already known depend on the introduction of surfactants and it is still necessary the utilization of high shear mixing equipments to guarantee that the dispersion will occur and remain stable under a minimum period, appropriate to the market (several months). Finally, as they are simple ferrites, it is not possible to vary and to control the saturation and magnetic remanence or the coercivity.
Many processes use the conventional method, or ceramic one, to synthesize ferrites. That one consists of a mixture of iron oxides and other metal oxides followed by steps of calcination and milling. The milling process may generate fine particles, however the granulometric distribution of the final product is very broad.
Hereafter we mention some known processes of the state of the technique:                The U.S. Pat. No. 6,811,718 describes a process that mix the oxide or precursor salts of the desired ferrite, containing specific surface area values in a well established range, until obtaining a slurry which is heated to high temperatures and then milled.        The U.S. Pat. No. 4,094,804 describes a process to disperse magnetic particles smaller than 15 nm in water. For so, they add a colloidal iron oxide solution to a mixture containing a salt of a fatty acid with 18 carbons or more. After that, the formation of a precipitate occurs and it is separated from the supernatant and dispersed in a medium containing an anionic surfactant with 8 to 30 carbons or a nonionic surfactant with 8 to 20 carbons.        The U.S. Pat. No. 6,767,396, U.S. Pat. No. 6,746,527, U.S. Pat. No. 6,726,759 reveal processes which yield a magnetic fluid with application in inks for MICR printers, from the dispersion of a magnetic iron oxide with average particle size smaller than 500 nm. For that they prepared a pre-dispersion by milling these particles in the presence of a dispersant. The ink is obtained from the mixture of this paste to more dispersing aids during processes of milling followed by some filtration steps.        The U.S. Pat. No. 5,958,282 (1999) describes a process which mixes non-magnetic iron oxide (Fe2O3) with water and a commercial surfactant, (Westvaco Reax 88B″), in a ball mill, under stirring, at a speed of 3500 RPM during 4 hours. The mixture is heated to 70° C. They obtained a magnetic fluid composed of magnetite particles with size in the order of 10 nm. The magnetic fluid presented magnetic saturation in the range 108-178 Gauss (approximately 8500 A/m).        The U.S. Pat. No. 6,048,920 (2000) describes in its process an ion exchange resin to capture iron sulfide particles which were then oxidized by a suitable base, to obtain resin bound magnetite nanoparticles. The obtained particles are in the range of 20-120 nanometers and presents saturation magnetization equal to 16.1 A/m.        The U.S. Pat. No. 4,452,773, also known as Molday Method, describes a well explored process of which there are already many modifications. The process is based on the precipitation of iron oxide in an alkaline solution containing a water-soluble polysaccharide, preferably dextran. The colloidal size composite particles comprise iron oxide crystals covered with dextran. As described, the size of the iron oxide particles obtained here is very heterogeneous.        The U.S. Pat. No. 4,951,675 (1990) describes a process for obtaining a biodegradable magnetic ferrofluid making some modification in the Molday method. Using a mean molecular weight dextran (75.000 Daltons) or a bovine serum albumin as covering materials, they prepared magnetic particle with sizes ranging from 1 to 500 nm. The use of centrifugation (1500 g for 15 minutes), dialysis (380 L of distilled water during three days, changing the water every day by each 80 ml of magnetic fluid) and ultrafiltration together allowed them to obtain a magnetic fluid whose average particle size was 120 to 150 nm, approximately.        The U.S. Pat. No. 4,109,004 describes a process for obtaining magnetic fluids from mixing petroleum sulphonate in aqueous medium followed by dissolution of iron salts, addition of the base and bubbling of carbonic gas. The mixture is stirred, heated and centrifuged. The supernatant obtained is a magnetic liquid.        The U.S. Pat. No. 5,500,141 describes a process for obtaining magnetic fluids for use in inkjet printers. For that, a Ni and/or Co substituted Mn—Zn ferrite was synthesized by the coprecipitation method followed by drying at 80° C. The utilization of Mn—Zn leads to an increase of the saturation magnetization and magnetic susceptibility which enables lower solid loadings and viscosity. The obtained powder was dispersed in a high shear mixer with the addition of a dispersant. The fluid was centrifuged and has saturation magnetization higher than 32 mT.        The U.S. Pat. No. 4,026,713 (1997) describes a process which mixes magnetite, glycerol, mono-lower alkyl ether of ethylene glycol and low molecular weight polyethylene diol, 200 g/mol, for obtaining a magnetic fluid composed of particles with sizes ranging from 5 to 30 nm and magnetic moment in the range of 25-30 A m2/Kg. This method also uses the centrifugation.        The U.S. Pat. No. 3,990,981 (1976) e U.S. Pat. No. 4,107,063 (1978) reveal a process for obtaining magnetic fluids to use as magnetic printing inks. The magnetic particles were covered with organic sulphates, sulphonates or amino carboxylates. They were then mixed with dispersants or surfactants. Both processes used centrifugation and took advantage just of the supernatant. The size of the magnetic particles was in the range 5-30 nm and presented magnetic moment equal to 20-25 A m2/Kg. So small size particles can generate the effect of super paramagnetism, which is not appropriate to the suggested application, unlike the product claimed in the present patent.        The U.S. Pat. No. 5,240,626 (1993) describes a process for obtaining an aqueous magnetic fluid based on magnetite, covering the magnetic particles with carboxy-functional polymers such as polymethacrylate. The particles were dried and re-dispersed in the desired medium with the aid of a surfactant or dispersant. Its size was in the range 2-20 nanometers and they presented a magnetic moment of 30 A m2/Kg.        The U.S. Pat. No. 4,161,454 describes a process for obtaining magnetite covered with copolymers, as powder, dried by a spray drier. The magnetic fluid consists of a mixture of this powder with a medium containing toluene and dispersants.        
Generally there are several methods for obtaining magnetic fluids, which are in consequence fluids with differentiated characteristics. The different methods aim to obtain not just simpler routes, but also enhance the properties responsible for a better performance in the applications for which they were developed.
The present invention uses special drying processes similar to the patents PI0901968-5 and PI0805592-2. However, after the functionalization step the material can be mixed directly with the solvent used in the preparation of the ferrofluid or be dried by special processes. The problem related to the direct mixture is the stoichiometric control of the fluid.
An important characteristic of the composites obtained in the present invention is their being nanoparticulate powders. As it is a material for extremely simple dispersion in the desired medium (water, for example), it is easy to obtain dispersions with different and well defined concentrations without addition of dispersants. As an example, 70 weight percent dispersions can be obtained without the aid of high shear mixers or special laminas.
For obtaining ferrofluids in aqueous medium from the dry powder, the control of the solids loading is complicated, because it needs the addition of other surfactants or dispersants that ensure the prolonged dispersion of the magnetic particles. The final product of the present invention is a dry powder of functionalized nanoparticulate ferrite, which can be added to compatible solvents, without needing extra addition of dispersants or surfactants.
The present invention enables obtaining both simple ferrites (MFe2O4 or MFe12O19) and mixed ferrites (Nx M(1−x) Fe2O4 or N1−y Mx+Y Fe(2−x)O4, for example) where M and N can be metals, such as Sm, La, Bi, Ba, Mo, Sr, Ni, Co, Ni, Fe, Mn, Cr, etc., through the coprecipitation method, functionalized by organic molecules containing carboxylic groups, which are polymers, or long chain acids or short chain acids, containing mono, di or tricarboxylic groups and/or alcohols, whose dispersion in polar or nonpolar media is improved. The present invention enables also obtaining ferrofluids, through the mixture of the obtained magnetic particles with an appropriate liquid carrier. The substitution of some elements in the ferrites may yield specific mechanical, optical and/or magnetic properties.